Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [RFC PATCH v3 2/4] arm64: Check the return PC against unreliable
 code sections
To:     Mark Brown <broonie@kernel.org>
Cc:     jpoimboe@redhat.com, mark.rutland@arm.com, jthierry@redhat.com,
        catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org,
        pasha.tatashin@soleen.com, linux-arm-kernel@lists.infradead.org,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
References: <65cf4dfbc439b010b50a0c46ec500432acde86d6>
 <20210503173615.21576-1-madvenka@linux.microsoft.com>
 <20210503173615.21576-3-madvenka@linux.microsoft.com>
 <20210504160508.GC7094@sirena.org.uk>
 <1bd2b177-509a-21d9-e349-9b2388db45eb@linux.microsoft.com>
 <0f72c4cb-25ef-ee23-49e4-986542be8673@linux.microsoft.com>
 <20210505164648.GC4541@sirena.org.uk>
From:   "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Message-ID: <9781011e-2d99-7f46-592c-621c66ea66c3@linux.microsoft.com>
Date:   Wed, 5 May 2021 13:48:21 -0500
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.7.1
MIME-Version: 1.0
In-Reply-To: <20210505164648.GC4541@sirena.org.uk>
Content-Type: text/plain; charset=windows-1252
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk


On 5/5/21 11:46 AM, Mark Brown wrote:
> On Tue, May 04, 2021 at 02:32:35PM -0500, Madhavan T. Venkataraman wrote:
> 
>> If you prefer, I could do something like this:
>>
>> check_pc:
>> 	if (!__kernel_text_address(frame->pc))
>> 		frame->reliable = false;
>>
>> 	range = lookup_range(frame->pc);
>>
>> #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> 	if (tsk->ret_stack &&
>> 		frame->pc == (unsigned long)return_to_handler) {
>> 		...
>> 		frame->pc = ret_stack->ret;
>> 		frame->pc = ptrauth_strip_insn_pac(frame->pc);
>> 		goto check_pc;
>> 	}
>> #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> 
>> Is that acceptable?
> 
> I think that works even if it's hard to love the goto, might want some
> defensiveness to ensure we can't somehow end up in an infinite loop with
> a sufficiently badly formed stack.
> 

I could do something like this:

- Move all frame->pc checking code into a function called check_frame_pc().

	bool	check_frame_pc(frame)
	{
		Do all the checks including function graph
		return frame->pc changed
	}

- Then, in unwind_frame()

unwind_frame()
{
	int	i;
	...

	for (i = 0; i < MAX_CHECKS; i++) {
		if (!check_frame(tsk, frame))
			break;
	}

	if (i == MAX_CHECKS)
		frame->reliable = false;
	return 0;
}

The above would take care of future cases like kretprobe_trampoline().

If this is acceptable, then the only question is - what should be the value of
MAX_CHECKS (I will rename it to something more appropriate)?

Madhavan